Composite alumina/metal powders, cermets made from said powders, and processes of production

Abstract

The invention relates to nano-composite powders of alumina and metal constituted of grains of micronic size. Each grain comprises a compact matrix of alumina of a specific surface area less than 5 m2/g, in which are dispersed crystallites of transition metals of alloys of these metals, of sizes less than 50 nm. The powder according to the invention may be produced starting with a precursor comprised of a mixed carboxylic salt of aluminum and one or more transition metals. The powders according to the invention permit producing by sintering cermets of alumina/metal benefitting from greatly improved mechanical and thermo-mechanical properties.

We claim: 
     
       1. A composite powder of ceramic/metal comprised of micron size grains of alumina and at least one transition metal, and wherein each micron size grain comprises a compact matrix, having a specific surface area less than 5 m 2  /g,   said matrix comprising α alumina (corundum) having a hexagonal structure,   said at least one transition metal being dispersed in each grain at the center of said alumina matrix in the form of crystallites of sizes less than 50 nanometers, and   the ratio by weight of metal/alumina being less than 30%.   
     
     
       2. A composite powder as in claim 1, and wherein said transition metal or metals are dispersed in said alumina matrix in each grain with a distribution of sizes such that 90% in number of the metallic particles have sizes distributed over a range of less than 8 nanometers. 
     
     
       3. A composite powder as in claim 2, and wherein said transition metal or metals are dispersed in said matrix of alumina of each grain with a distribution of sizes such that 90% in number of the metallic particles have sizes comprising between 1 and 7 nanometers. 
     
     
       4. A composite powder as in claim 1, and wherein said grains comprise a matrix of alumina and at least one metal selected from the group consisting of iron, chromium, molybdenum, cobalt, nickel, niobium or an alloy of these metals. 
     
     
       5. A composite powder as in claim 1, said grains comprising a matrix of alumina and at least two transition metals, dispersed in the matrix in the form of a metal alloy. 
     
     
       6. A composite powder as in claim 5, and wherein the grains contain crystallites of an alloy selected from the group consisting of iron-chromium, nickel-cobalt or nickel-chromium. 
     
     
       7. A cermet produced by sintering a composite ceramic/metal powder comprised of micron size grains of alumina and at least one transition metal, each grain comprising a compact matrix, of a specific surface area less than 5 m 2  /g, and said matrix being comprised of α alumina of a hexagonal structure, said transition metal or metals being dispersed in each grain at the center of the matrix of alumina in the form of crystallites of sizes less than 50 nanometers, and the ratio by weight of metal/alumina being less than 30% said cermet having a bending resistance of between 500 and 1000 megapascals and a resilience of between 5 and 10 megapascals per √m.